Since an organic EL display device can perform high luminance display by spontaneous light, the organic EL display device is suitable for small display screen. Therefore, the organic EL display device is paid attention as the next generation display device to be mounted on such as a portable telephone set, a DVD player, a PDA (personal digital assistance). An organic EL element (referred to as “OEL element”, hereinafter) is current driven in order to solve the problem of luminance variation.
In an organic EL display panel of the organic EL display device for a portable telephone set having the number of terminal pins corresponding to 396 (=132×3) column lines and the number of terminal pins corresponding to 162 row lines has been proposed and the numbers of the terminal pins for the column lines and the row lines tend to be increased.
The OEL element has capacitive load characteristics. Therefore, when a positive matrix type OWL element is current-driven, a peak current is generated to initially charge the OEL element and a current drive circuit having a current output stage for generating a peak current is known (Patent Reference 1). The luminance control, that is, the grey level control, of an OEL element in such kind of current drive circuit is performed by controlling a drive current.
On the other hand, in an active matrix type organic EL drive circuit, a capacitor of a pixel circuit stores a drive current as a voltage value and there are various systems therefor. One of these systems is the time-division grey level control system. When the number of bits for grey control is, for example, 6 in the time-division grey level control system, one frame is divided to 6 sub frames having different drive times and the grey level control is performed time-divisionally by driving the OEL elements in predetermined time periods obtained by combining 6 sub frames in the frame corresponding to grey levels at a constant voltage. That is, the luminance is controlled by not the drive current value but the drive time.
An OEL element drive circuit, in which OEL elements arranged in matrix are current driven and are reset by grounding anodes and cathodes of the OEL elements is disclosed in Patent Reference 2. Further, a technique for current driving OEL elements with low power consumption by using a DC-DC converter is disclosed in Patent Reference 3.
Patent Reference 1: JPH11-45071A
Patent Reference 2: JPH9-232074A
Patent Reference 3: JP2001-143867A